Adolescent men who have sex with men (AMSM) account for almost 70% of HIV diagnoses among all young people, and are alone in facing increasing incidence in HIV/AIDS. Despite this disproportionate burden, current HIV prevention programs focus primarily on adults and heterosexual youth. As such, there is an urgent need for evidence-based HIV prevention programs targeting AMSM. Because issues affecting sexual health decisions among AMSM are unique, intervention programs cannot be translated from heterosexually focused interventions. Instead, they need to be designed from the ground up to ensure appropriately tailored content that resonates with the target population. It is unlikely tha a 'single bullet' will be able to affect population-level behavior change necessary to reverse concerning recent trends. Instead, we should aim to develop an arsenal of HIV preventive tools available to AMSM across a myriad of access points. With over half (54%) of adolescents (of all races, and across all income levels) text messaging every day, this mode of communication represents a novel opportunity to deliver intervention material where youth 'are', in an engaging and proactive fashion. Combined with online recruitment, this approach can overcome access barriers to engagement of AMSM in HIV prevention. Recent use of text messaging to successfully deliver behavior change interventions provide optimism for its inclusion as an access point for AMSM-targeted HIV prevention. We propose to address the paucity of evidence-based HIV prevention program options for AMSM with Guy2Guy (G2G), a novel text messaging-based HIV prevention program designed specifically for AMSM ages 14-18. Our guiding theoretical model is the Information-Motivation-Behavioral Skills (IMB) Model of HIV Preventive Behavior. We will achieve the following aims: Specific Aim 1: Iteratively design Guy2Guy (G2G), a 6-module, text messaging-based HIV prevention program tailored and culturally appropriate for AMSM in the United States. We will conduct focus groups with AMSM (n=48) to confirm program components (e.g., social support via Text Buddy, optimal time of delivery across the day) and saliency of intended program topics. Program messages will then be developed with the ongoing participation of AMSM in our Youth Advisory Council (n=20). Finally, we will conduct a functionality test of the programmed messages and intervention protocols in a Beta test of AMSM (n=10). Specific Aim 2: Test G2G intervention feasibility, acceptability, and preliminary efficacy in a pilot randomized controlled trial (n=184) among AMSM across the United States. Youth will be randomly assigned to either the G2G intervention or the control group, a didactic, untailored text messaging program that provides basic HIV facts readily accessible online. Our main outcome measures will focus on feasibility (e.g., recruitment and retention rates) and acceptability (e.g., Text Buddy, SOS Tonight). Our primary efficacy outcome measure will be frequency of unprotected sex acts at 3-months post-intervention.3, 16 We have two secondary efficacy outcomes: 1) sexual abstinence, and 2) increased HIV testing rates following the CDC's recommendation of twice annual HIV testing for MSM.